Heretofore, a variety of circuitry of the uninterrupted power supply units have been proposed as disclosed in, for example, Japanese Patent Laid-open Publication JP 01-222635 (referred to as Patent Document 1) and Japanese Patent Laid-open Publication JP 08-223822 (referred to as Patent Document 2).
One conventional uninterrupted power supply unit shown in Patent Document 1 is provided with a constant voltage/constant frequency power supply unit (CVCF) in which an alternative current (AC) input voltage is once converted into a direct current (DC) voltage, then the converted DC voltage is reversely converted into the AC voltage again, and a bypass circuit comprising semiconductor switches which bypass the CVCF. Such power supply unit is constituted so that in a normal voltage condition as well as in a dropped voltage condition the AC is once converted into the DC through a converter and the DC is again converted into the AC with an inverter. As a result, the current is always passed through semiconductors even in the normal voltage condition, causing to give rise to a loss of power, to reduce a total efficiency of the whole unit, and to upsize a cooling device. There also exists a problem that since an output of the inverter requires PWM controlled-rectangular wave, a filter for filtering the output wave must be upsized.
Another conventional uninterrupted power supply unit shown in Patent Document 2 is constituted so that in normal condition a commercial line is directly connected through the straightforward switch to the load, but in the case where a voltage of the commercial line is decreased to less than a given value, the straightforward switch is cut off and a battery power is supplied to the load through the inverter and a step-up transformer. In such configuration, since the step-up transformer is required to have a function for filtering a rectangular voltage generated in the inverter, and to transmit a voltage having a commercial frequency, the problem exists in that the product of voltage and time, that is, flux volume, is required to be a large value, resulting in the system of upsized and expensive construction.
Patent document 1:
    Japanese Patent Laid-open Publication JP 01-222635Patent document 2:    Japanese Patent Laid-open Publication JP 08-223822